I. Field
The present invention relates generally to communication, and more specifically to pilot transmission and channel estimation in a wireless multi-antenna communication system.
II. Background
A multi-antenna communication system supports (1) multiple-input multiple-output (MIMO) transmission from multiple (T) transmit antennas to multiple (R) receive antennas and (2) multiple-input single-output (MISO) transmission from multiple transmit antennas to a single receive antenna. A MIMO channel formed by the T transmit antennas and R receive antennas is composed of S spatial channels, where S≦min {T, R}. The S spatial channels may be used to transmit data in parallel to achieve higher overall throughput and/or redundantly to achieve greater reliability. A MISO channel formed by the T transmit antennas and single receive antenna is composed of a single spatial channel. The T transmit antennas may be used to transmit data redundantly to achieve greater reliability.
An accurate estimate of a wireless channel between a transmitter and a receiver is normally needed in order to recover data sent via the wireless channel. Channel estimation is typically performed by sending a pilot from the transmitter and measuring the pilot at the receiver. The pilot is made up of symbols that are known a priori by both the transmitter and receiver. The receiver can thus estimate the channel response based on the received symbols and the known symbols.
The multi-antenna system may concurrently support both MISO receivers (which are receivers equipped with a single antenna) and MIMO receivers (which are receivers equipped with multiple antennas). MISO and MIMO receivers typically require different channel estimates and thus have different requirements for the pilot, as described below. Since pilot transmission represents overhead in the multi-antenna system, it is desirable to minimize pilot transmission to the extent possible. However, the pilot transmission should be such that both MISO and MIMO receivers can obtain channel estimates of sufficient quality.
There is therefore a need in the art for techniques to efficiently transmit a pilot in a multi-antenna system.